Photographic tanning developer



United States Patent PHOTOGRAPHIC TANNING DEVELOPER Jozef Frans Willems, Wilrijk-Antwerp, and Albert Emiel Van Hoof, Bei'chem-Antwerp, Belgium, assignors to Gevaert Photo-Producten N.V., Mortsel, Belgium, :1 Belgian company No Drawing. Filed. Sept. 2, 1958, Set. No. 758,576

Claims priority, application Netherlands Sept. 5, 1957 6 Claims. (Cl. 96-66) The present invention relates to a process for inhibiting the aerial oxidation of a tanning developer, and more particularly to a process whereby this oxidation is inhibited by the use of a stable anti-oxidizing agent which does not compete with the tanning developing substance.

It is known to develop photographic material provided with an unhardened or little hardened light-sensitive silver halide emulsion layer, with gelatin as binding agent, after exposure to an image in a so-called tanning developer. The oxidation products of such a developer strongly harden gelatin. On developing, a hardening of the gelatin by the above-mentioned oxidation products takes place at the same time on the areas where silver is' formed. The non-developed and. thus unhardened or little hardened parts of the emulsion layercan be removed with warm water, whereafter a hardened gelatin relief image is left.

In most cases, polyhydroxybenzenes, especially pyrocatechol and pyrogallol, both in alkaline medium, are used as tanning developer substances in photography. The drawbacks of such developers are their instability and their quite rapid darkening and uselessness by aerial oxidation. Sodium sulphite, generally known as a very fiective agent against aerial oxidation of developers cannot be used or at least not in the necessary amount in a tanning developer since the formed oxidation products of the developing substance react with the sodium sulphite and not with the gelatin, so that the latter is not hardened anymore on the exposed areas, with the consequence that no relief image can be obtained anymore; in most cases, the tanning developers are prepared just before use and discarded after use.

It is also known to use ascorbic acid (vitamin C) as anti-oxidizing agent in different developers (British specification 560,371) and also as anti-oxidizing agent in tanning developers (U.S.P. 2,415,666). This process, however, involves many drawbacks. First, ascorbic acid is unstable in alkaline medium which results in a strong decrease of the anti-oxidizing action. Further, it has been. stated that after exposure and development of a photographic element in a tanning developer to which ascorbic acid was added as anti-oxidizing agent, the relief image, on removing the non-developed unhardened gelatin, is insufliciently hardened and is either completely or partially washed away by the warm water. This occurs in particular when relatively large quantities of ascorbic acidwhich owing to the instability of ascorbic acid in alkaline medium are necessary to keep useful the tanning developer for a sufliciently long periodare added to the developer,

This phenomenon can be explained as follows: ascorbic acid itself is a developing substance and competes as such with the tanning developer substance. Since the oxidation products of ascorbic acid do not harden gelatin, the hardening is insutficient on the exposed areas where the ascorbic acid has likewise formed black silver by com; petition with the tanning substance. As a result, the exposed areas which are developed, but insutficiently hardice I enedbythe ascorbic acid, are either completely or partly dissolved by the warm water on eliminating the unex-- posed and thus in the development unhardened gelatin parts. V

.It is an object of the present invention to provide a photographic tanning developerwhich is stable to aerial tilling the water under reduced pressure.

oxidation. A further object is to provide a novel antioxidizing agent which does not compete with the tanning developer substance used.

According to the present invention these objects are accomplished by the presence, in a silver halide tanning developer solution comprising an alkaline solution of a polyhydroxybenzene, of a water soluble salt of an ascorbic acid borate. It was stated, indeed, that the as.- corbic acid borates contrary to the free ascorbic acid, are very stable in alkaline medium where they act as their corresponding salts, and moreover that theydo not compete with the tanning developer substance used.

Ascorbic acid borate is prepared by esterifying ascorbic acid with boric acid. When the esterification is carried out in the presence of a base, salts of ascorbic acid borate are obtained. In this way, potassium ascorbic acid borate can be obtained by heating'to 70 C. 2 mol eq. of ascorbic acid with 1 mol eq. of boric acid and 1 mol eq. of potassium hydroxide in aqueous solution and by dis- When in this preparation, other hydroxides of alkali, metals are used, such as sodium hydroxide, or hydroxides of alkalineearth'metals, such as barium hydroxide, or still other hydroxides, such as ammonium hydroxide, the corresponding salts of ascorbic acid borate can be prepared.

It appears from the measurement of their alkali consumption, that these ascorbic acid borates are much more stable than ascorbic acid. This alkali consumption of ascorbic acid increases steadily with time, which points. to the decomposition ofascorbic acid intoother products. Simultaneously the'reduction power decreases. The alkali consumption of ascorbic acid borates on the contrary is constant for a long period and finally increases slightly, which points to a muchlarger stability since at the same time the reduction power remains on the same level.

A further great advantage of the ascorbic acid borates in respect of ascorbic acid: is. the very strongly reduced competition with the present tanning developer substance duringdevelopment.

The technical progress obtained by the application of the process according to the present invention can easily be derived from the results of the tests mentioned hereinafter.

5 developers (A, B, C, D and E) are prepared with the If /2 litre of each of these developing solutions is poured into an open wide-necked flask, 'so that about 50 cm. of liquid surface is exposed to the air, solution A is coloured dark brown already after some hours. For solution B, this step lasts about 24 hours, whilst for the solutions C, D and E several days are necessary before they become dark brown by aerial oxidation.

A photographic paper provided with a light-sensitive silver halide emulsion is exposed to an image and divided into three strips. Each strip is immersed for one minute at 20 C. in developers C/D, and E resp. On washing in warm Water, the developed areas obtained with the rel nted Jan. 10, 1,9 1

3 solutions C and E are almost completely washed away whilst with solution D a sharp relief image is obtained.

The results of the above-mentioned steps accentuate also the surprising character of the present invention as herefrom appears that when ascorbic acid and boric acid are added as such to the tanning developer (solution E), the results attained are much worse than on the use of the ascorbic acid borates (solution D) and are wholly to compare with the results attained if ascorbic acid (solution C) alone is added.

It is evident that the developers which stability to aerial oxidation is increased by the application of the process according to the present invention, can possess besides the tanning developer substance and the ascorbic acid borate, still other usual bath ingredients such as for instance wetting agents and sequestering compounds.

The following examples illustrate the present invention without limiting, however, the scope thereof.

Example I A contrasty light-sensitive silver halide emulsion on a paper support is exposed to an image and developed for 1 minute at 20 C. in the following bath:

G. Potassium salt of ascorbic acid borate 5 Pyrocatechol 6 Sodium carbonate (anhydrous) 30 Water to 1 litre.

After washing in water at 35-40 C., a sharp negative relief image of the original is obtained.

Example 2 A contrasty light-sensitive silver halide emulsion on a paper support is exposed to an image and developed for 1 minute at 20 C. in the following bath:

Water to 1 litre.

After washing in water at 35-40 C., a sharp negative relief image of the original is obtained.

Example 3 A contrasty light-sensitive silver halide emulsion on a 4 paper support is exposed to an image and developed for 1 minute at 20 C. in the following bath:

Water to 1 litre.

After washing in water at 35-40 C., a sharp negative relief image of the original is obtained.

Example 4 A contrasty light-sensitive silver halide emulsion on a paper support is exposed to an image and developed for 1 minute at 20 C. in the following bath:

G. Potassium salt of ascorbic acid borate 5 Pyrocatechol 6 Potassium hydroxide 2 Ethylenediamine tetracetic acid 1 Water to 1 litre.

After washing in water at 35-40 C., a sharp negative relief image of the original is obtained.

We claim:

1. A photographic tanning developer solution having reduced tendency to aerial oxidation and capable of forming relief images upon development of an exposed silver halide emulsion, comprising an alkaline solution of a polyhydroxybenzene selected from the group consisting of pyrogallol and pyrocatechol and a water-soluble salt of a boron ascorbic acid compound which is the reaction product of boric acid and ascorbic acid.

2. A developer as defined in claim 1 wherein the polyhydroxybenzene is a pyrogallol.

3. A developer as defined in claim 1 wherein the polyhydroxybenzene is pyrocatechol.

4. A developer as defined in claim 1 wherein the salt is an alkali-metal salt.

5. A developer as defined in claim 1 wherein the salt is an alkali-earth metal salt.

6. A developer as defined in claim 1 wherein the salt is an ammonium salt.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PHOTOGRAPHIC TANNING DEVELOPER SOLUTION HAVING REDUCED TENDENCY TO AERIAL OXIDATION AND CAPABLE OF FORMING RELIEF IMAGES UPON DEVELOPMENT OF AN EXPOSED SILVER HALIDE EMULSION, COMPRISING AN ALKALINE SOLUTION OF A POLYHYDROXYBENZENE SELECTED FROM THE GROUP CONSISTING OF PYROGALLOL AND PYROCATECHOL AND A WATER-SOLUBLE SALT OF A BORON ASCORBIC ACID COMPOUND WHICH IS THE REACTION PRODUCT OF BORIC ACID AND ASCORBIC ACID. 